(1) Field of the Invention
The present invention relates to a synchronizer, and in particular, to a digital audio synchronizer for compensating an audio signal for a time difference between the audio signal and an associated video signal whose timing is disturbed by being synchronized to a reference video signal.
(2) Description of the Prior Art
In a multi-station TV broadcasting system, a video signal from a local station is frame-synchronized to a reference signal of the central station. The frame synchronization causes a time delay in the video signal, so that a time difference results between the synchronized video signal and an audio signal accompanying for this video signal. In order to compensate the time difference, a digital audio synchronizer or a digital audio-delay device is used.
In a known digital audio synchronizing system, the audio signal is digitalized and then stored in a memory. The stored signal is then read out from the memory. Timewise, the read-out address is offset from the write-in address by a number of address locations corresponding to the time difference of the reference signal from the video signal. Therefore, the output audio signal is delayed from the incoming audio signal by the same time difference as exists between the reference signal and the incoming video signal. Accordingly, the audio signal is synchronized to the video signal which in turn is synchronized to the reference signal.
In the known audio synchronizer, any variation or change in a video delay between the incoming video signal and the reference signal results in a jump in the read-out address. This produces a discontinuity in the output audio signal and ensuing unacceptable click sound.
In order to resolve such a problem, the present inventors already proposed another digital audio synchronizing system in UK patent application GB No. 2 160 393A (corresponding to a copending U.S. application Ser. No. 720,813 filed on Apr. 8, 1985 and assigned to the same assignee) where a digital audio signal read out from memory addresses corresponding to a previous video delay before occurrence of variation in the video delay is cross-faded with another digital audio signal read out from memory addresses offset by the video delay variation, for a predetermined time period.
However, in such an audio synchronizer, if both read-out audio signals having delays before and after occurrence of the video delay variation are equal in the frequency but inverse in the phase, the cross-faded output audio signal has an undesirable waveform which comprises a gradually reducing amplitude portion and a subsequent increasing amplitude portion as if an audio signal is amplitude-modulated, as will be made clear in later description.